1. Field of the Invention
The present invention generally relates to the field of optical memories and apparatus for recording and reading an optical memory, and more particularly to an optical memory and apparatus capable of recording and reading data quickly and in a more stable manner.
2. Description of the Related Art
Optical memories having a layer or film of semiconductor materials are widely used in industrial and consumer recording systems such as video recorders, digital audio recorders, and document digital recorders. A signal is recorded on the optical disc by irradiation with a laser beam to form bits of information in the layer of semiconductor materials. The optical disc permits direct reading after the information has been written or recorded and random access to the information. Optical discs may also be used for erasable recording.
There are several different techniques by which information may be recorded using optical discs. These techniques include heat application (i.e., ablation or other means of changing disc topography), chemical reaction, particle coalescence, phase change, and magnetization change. For the first four techniques, the optical contrast is obtained by the difference in reflectivity between the recorded and unrecorded states. In magnetization change, the contrast is obtained from the change in the direction of the polarization of light due to the change in the polarity of magnetization in a recorded area.
These techniques are known in the art and will not be discussed in detail here. Briefly, ablation utilizes laser light to selectively melt the recording medium. The surface tension of the molten liquid pulls the film away from the center, resulting in the formation of a hole or pit. Optical media on which data is recorded by a topography change include bubble forming media. A bubble is formed due to either gas evolution from the underlying polymer or microswelling of the metal or polymer layer upon laser irradiation. In chemical reaction recording, a laser-induced chemical reaction is initiated between two initially discrete layers. In particle coalescence, a very thin and discontinuous metal film is subjected to laser irradiation to induce a coalescence of metal particles. In phase changes, the optical properties of the recording medium are changed by inducing phase changes therein. Depending on the laser characteristics, the recording medium exhibits two different reflectances and the change between the two states may be reversible. The reversible change may, for example, be an amorphous to crystalline phase transition. In magnetization, the area irradiated with the laser exhibits a changed direction of magnetization. A linearly polarized laser light of low intensity is used to sense the change in magnetic direction.
So-called image filing systems are widely used as business machines for recording and reproducing document data. As is known in the design of such systems, image data is first optically read off a document, and then the read out image data is recorded into a recording medium. The image data recorded onto the optical memory may be read therefrom and subsequently reproduced on a display unit for visual presentation or supplied to a printer for printing a hard copy.
Optical disc recording devices used in these known image filing systems employ optical discs for recording image data. Image data is recorded in spiral tracks on the surface of the optical disc. For recording or reading out the image data, an optical head, set close to the optical disc, is driven by a linear motor to rectilinearly move in the radial direction of the optical disc.
Two methods are conventionally used for recording or reading the image data; one is a so-called constant linear velocity (CLV method), and the other is a so-called constant angular velocity (CAV method). In the CLV method, a track on the optical disc moves at a constant linear velocity relative to the optical head which has been moved to a particular radial location on the disc. Consequently, the optical disc is rotated at a decreasing angular velocity as the optical head is moved to a recording location further removed from the center of the disc; that is, angular velocity of the disc is caused to decrease with increasing radius or radial position of the optical head. In the CLV method, for recording and reading out image data, the rotating speed of the optical disc changes as the position of the optical head changes in its radial path above the optical disc in order to maintain linear velocity constant with increasing radius. The CLV method ensures that all tracks of the disc move at a constant speed relative to the optical head. However, it takes a long time for the angular velocity to settle to a constant value in each access at different radial positions of the head. Thus, the CLV method requires a long access time and exhibits a slow data transfer rate.
In the CAV method, the rotating speed of the optical disc is set at a constant value for stabilizing the record/read out operation and reducing an access time. However, since the angular velocity of the optical disc is constant, image data becomes less dense with increasing radius. Thus, the CAV method does not lend itself to the production of high density optical discs.
Some improvements are proposed for the CLV method. One is a method where the rotating speed is set at a constant value. The frequency of a clock used in the system is varied in accordance with the head position for recording and reading data so that the data are formed on the disc at a predetermined constant spacing therebetween along the tracks. Hereinafter, this method will be referred to as a constant linear density method (CLD method).
In the CLD method, the frequency of the clock increases as the optical head moves from the inside to the outside of the optical disc. It is difficult, however, to control the high frequency of the clock at the outside of the disc.
On the other hand, the optical disc is apt to have defect portions caused during data storage and retrieval or during manufacture. One known technique for detecting defect portions on an optical disc during storage and retrieval from the disc is disclosed by U.S. Pat. No. 4,835,757 incorporated herein by reference. It is not economical to throw away optical discs found to contain a small percentage of defective recording area.